At high concentration (50 micrograms/ml), diallyl trisulfide (DATS) had an inhibitory effect on T cell activation (compared with control group, P < 0.05). But at appropriate concentrations (3.125-12.5 micrograms/ml), DATS augmented the activation of T lymphocytes by Con A (compared with control group, P < 0.01). The augmentation of T cell activation by DATS was related to its inhibitory effect on the production of nitric oxide (NO) by macrophages. In a wide range of concentrations (1-100 micrograms/ml), DATS can inhibit the production of NO by macrophages (P < 0.05, P < 0.01). In addition, DATS can antagonize the inhibition of tumor-derived immunosuppressive factors produced by S180 cells and Ehrlich ascitic cancer cells on the activation of T cells, and reduce the inhibitory rate significantly (P < 0.01). DATS, despite its inhibition of the production of NO by macrophages, can significantly enhance the production of hydrogen peroxide (H2O2) by macrophages. When macrophages were pretreated with DATS for 24 h, the cytotoxicity % of macrophages to three tumor cell lines was significantly higher than that in corresponding control group (P < 0.05, P < 0.01). In the presence of both DATS and LPS, the cytotoxicity of macrophages was further enhanced so that the cytotoxicity % of macrophages to tumor cells was significantly higher than either that in the presence of DATS alone or that in the presence of LPS alone (P < 0.05, P < 0.01). These results indicate that DATS can augment the activation of T cells and enhance the anti-tumor function of macrophage, suggesting that DATS may be potentially useful in tumor therapy.